The field of invention is generally related to mass and thermal transfer in a manner that is particularly useful in blood oxygenators and other medical devices.
A brief history and present status of relevant technology is appropriate to understanding the subject invention with regard to medical applications. There has been a need to utilize principles of mass and heat transfer more effectively in, for example, oxygenators and dialyzers.
For example, heart-lung machines, which utilize oxygenators, are employed during surgery in the USA approximately 300,000 times per year. Renal dialysis is performed approximately 750,000 times per week on 250,000 people to sustain the life in the presence of kidney failure. These examples in medicine and a score of others in industry attest to the need for compact devices which permit the transfer of gases, solutes, liquids, and/or heat across metallic and synthetic surfaces. Especially in medical applications, efficiency (transfer rates per unit surface area, per unit volume, per unit flow, and per unit priming volume) is critically important because of limited flow rates and small working volumes characteristic of human physiology.
The typical heart-lung machine consists of a complex interlinkage of 4 to 6 separate components, including a pump, oxygenator, heat exchanger, flow meter, and dynamic reservoir; the components being connected together by plastic tubing through which blood and water flow (see FIG. 1). The disadvantages of the present heart-lung machines are several. They include an inordinately large priming volume. Priming volume refers to the amount of blood or blood substitute required to fill the device prior to use. It is important to reduce the priming volume to minimize the danger of AIDS and hepatitis. Economic costs of blood and fluid are also important reasons to reduce the priming volume of extracorporeal circuits. Normal adults can typically only tolerate the loss of 500 to 1,000 ml (milliliters) of blood during operation, but cannot sustain losing an additional 1,250 to 2,500 ml to prime the device. It is therefore common for patients undergoing open heart surgery to receive multiple transfusions of blood. The cost of filling the priming volume with a mixture of blood or blood substitutes can be as much as $1,000 (blood processing, saline solution, plasma, hydroxy ethyl starch cost between $40 and $330 for each 500 ml required).
Inefficient mass transfer of oxygen and carbon dioxide is also a problem in heart-lung machines. Membrane surface areas of 2 to 4 M.sup.2 (square meters) are typically required, necessitating high expense and large priming volumes. The hundreds of thousands of commercial membrane oxygenators and heat exchangers used each year have oxygen transfer rates of 70-100 cc/M.sup.2 /min (cubic centimeters per square meter per minute) and heat transfer rates of 2 to 4 cals/cm.sup.2 /C..degree./min (calories per square centimeter per degree centigrade per minute). These rates are essentially the same for all available commercial devices except for small variations achieved by induction of low levels of turbulence by alteration of flow paths. As will be explained, the efficiency of all current devices is limited by the boundary layer effect as it influences the operation of the Fick diffusion equation at the diffusion surface.
Yet another problem with heart-lung machines is the risk of air embolism from the reservoir. The dynamic (see below) blood reservoir is open to the atmosphere, thus requiring constant human monitoring to prevent massive air embolism. Moreover, the reservoir typically accounts for 500-1500 ml of total priming volume, and exposes blood protein to damage due to surface free energy at the air-fluid interface.
Another drawback is the operational complexity induced by multiple interdependent components connected by plastic tubing, as shown in FIG. 1.
Also of concern is the expense resulting from the cost of the 4 to 6 disposable components which comprise the heart-lung machine, which cost from $100 to $450 per item. Currently, practices typically cause the disposal of these items after each surgery.
Finally, expensive hardware is needed for monitoring the blood flow rate through the heart-lung machine system. Expensive electromagnetic or Doppler flow meters are used with centrifugal pumps.
Current devices for renal dialysis suffer analogous impediments. Limited mass transfer rates require that patients be attached to artificial kidneys for 4 hours three times per week to sustain life.
The above-mentioned disadvantages are only a fraction of the problems encountered by existing technology.
Over twenty (20) years ago, one of the subject inventors reported on the impediment to diffusion created by the boundary layer adjacent to the diffusing surface, Maloney, J. V., Brown, G. E., Van de Water, J. M., Lee, W. H., Pall, D. B. Boundary Layer in Membrane Oxygenators, 18 Surg. Forum 134-136 (1967). This 1967 work, like the efforts by many other investigators, failed to produce a practical device, in part due to the heat generated by intermolecular friction in the fluid which was detrimental to red blood cells. The flat surface, despite rotation, was so inefficient as a diffusion surface that the energy required produced more heat than was practical to dissipate. Accordingly, this work was deemed unsuccessful and was abandoned. The substitution of microporous hollow fibers for solid membranes a decade ago reduced from 4 to 2 M.sup.2 the membrane surface area required in oxygenators. Otherwise, in the 25 years since the 1967 article, little has been done to further the technology. Commercial endeavor has been directed in the past 15 years to altering the flow path between hollow fibers in an attempt to induce turbulence and increase diffusion. The efforts have produced minor improvements in gas transport in the range of 10 percent. A similar lack of progress characterizes the current status of diffusion rates through wettable fibers in renal dialysis.
Accordingly, there has been need for an effective and practical means for improving mass transfer in compact diffusion devices. The subject invention achieves mass and heat transfer rates never before obtained.